Secondary immune organs, such as lymph node, tonsil and spleen, are highly structured tissues which dynamically change mechanical and biological functionality in response to antigens. Of particular importance is the activation of naïve B cells in these lymphoid tissues to form sub-anatomical structures, germinal center (GC), that program B cell conversion into antibody producing cells, which represents a powerful defense mechanism against pathogens and is characterized by immunological memory. To date, we have relied heavily on live animal models to understand immune cell development, functioning, and screening of immunotherapies against diseases, but such approaches are costly with long turnaround times. We are yet to fully understand how the cell microenvironment in lymphoid organs facilitates the GC reaction that leads to the production of antibodies. Therefore, ex vivo engineered B cell organoids could offer a new approach for studying GC B cell physiology and pathology, and potentially hematological malignancies of B cell origin, as well as screening of therapeutics including immunotherapeutics.
From an anatomical perspective, secondary lymphoid organs are composed of supporting cellular compartments, including B cell and T cells, that work together to orchestrate adaptive immune responses. B cell follicles are composed of a dense stromal network of B cell activating follicular dendritic cells (FDCs) and Arg-Gly-Asp (RGD)-presenting ECM. Activation process requires interactions between antigen-primed B cells and follicular helper T (TFH) cells via a CD40L ligand, and secretion of IL-4. GC B cells are naturally prone to apoptosis unless rescued by anti-apoptotic signals. Although activation of B cells can be achieved through stimulation with antibodies (anti-Ig or anti-CD40), CD40L, lipopolysaccharide and cytokines, such as IL-4, in vitro, the resulting cell growth is transient with poor cell survival and previous efforts did not investigate the GC reaction in the context of lymphoid tissue microenvironment, and have not shown the ability to modulate the extent of GC reaction. Combinations of 3D scaffolds and engineered stromal cell lines for generating artificial secondary lymphoid organs have also been made, however these scaffolds have shown GC formation only when implanted in vivo by exploiting the host microenvironment. Thus, there is an ongoing and unmet need for alternative secondary immune organs that can be created and used in an ex vivo context.